The IEEE-1394 limits the available cable length to 4.5 meter. Fortunately, AS5643 extends obtainable length significantly by adding transformers as well as defines the types of cables and connectors. Still, quite often this is not enough. Then repeaters are very useful to add another cable run. Repeaters can help as they typically use active 1394 PHY layer devices. The IEEE-1394 specification limits the total number of devices on a single 1394 bus to 63, the number of devices in a single chain to 16 as well as the available bus power (not applicable if the repeaters are self-powered which is typically the case in AS5643 implementations). For in-line extensions repeaters are typically have a pass-through configuration, i.e. connectors to either side of the enclosure connected to different ports on the same PHY.
Another interesting usage mode is to use the repeater as node replacements. Assume you are building a complex A&D system. And further assume that - while your system is already built - a single device is missing (not available yet or missing because of damage/repair/...). Depending on your network configuration that can result that some devices on your bus might no longer be connected to the rest. However, when using the repeater as a node replacement the bus integrity can be maintained on a physical level. Granted, the repeater will have no higher-level functionality but at least all nodes are still connected. In this case a pass-through connector configuration is not ideal and is better replaced with running multiple port connections through a single 38999 connector.